Red Oil Light

This past weekend, I was doing some work on my 2002 330xi. I've got 181,XXX miles on it. One of the things on my list was an oil change. I have done these myself in the past, so it was nothing big. After this change though, something seemed to go wrong. I started the car back up and let it warm up. On my test drive, the Low Oil Pressure light came on (red oil light). At this point I was able to pull back in the garage and shut it down right away. I did notice, however, that the light only came on at idle RPMs (about 600rpm). I figured something might have gotten in the oil system or some sludge, so I drained that oil and put another fresh 7 qts in. I let it warm up and gave the same light. I am using Mobil1 0W-40 with an OEM oil filter. The car is stock and nothing else was changed with the oil system during my work session. I bought some Castrol 5W-30 synthetic to try next time, but won't get the opportunity to work on it for 2 weeks. Any ideas or suggestions would be greatly appreciated.